Method of continuously casting bars for preventing distortion during solidification of the bars

ABSTRACT

A continuous casting operation for forming bars is comprised of pouring molten steel into the inlet end of a mold chamber in which the bar is formed and from which the bar is withdrawn while it still has a liquid core. Within the mold chamber the molten steel forms a liquid level, and a first section of the chamber having converging walls commencing at the liquid level and extends downwardly for applying a symmetrical cooling effect to the molten steel and, at the same time, constraining the outer surface of the bar as it contracts and develops a solidified outer layer or crust. The constraint afforded by the walls of the first section prevents a distortion in the shape of the bar as it begins to solidify. From the first section of the chamber the bar passes through a second section where the cooling operation is continued but without any constraint so that a gap develops between the bar and the walls of the chamber. After passage through the second section the bar may be withdrawn from the outlet end of the chamber or, alternatively, it may pass through another section wherein increased cooling is effected and then, if necessary through a further section where the cooling effect is decreased as in the second section. From the outlet the bar is guided to rollers which withdraw it from the mold chamber. Before reaching the rollers, cooling means, such as water sprays, are provided for cooling the casting. The mold chamber is provided with means for circulating a coolant fluid for removing heat from the molten steel within the mold chamber.

United States Patent [72] Inventors Werner Staufl'er, Armin Thalmann,Uster,

and Carl Kueng, Emmenlirucke [21] AppLNo. 693,884 [22] Filed Dec. 27,1967 [45] Patented Feb. 16,1971 [73] Assignees Aktiengesellschaft DerVon Moos, Schen Eisenwerke Luzern,; Concast AG Zurich, Switzerland [32]Priority Dec. 28, 1966 [33] Switzerland 18686/66 [54] METHOD OFCONTINUOUSLY CASTING BARS FOR PREVENTING DISTORTION DURINGSOLIDIFICATION OF THE BARS 6 Claims, 2 Drawing Figs.

[52] U.S.Cl. 164/82, 164/273 [51] Int. Cl. ..B22dll/00 [50]FieIdofSearch ..164/82,89, 273,282, 283,85

[56] References Cited UNITED STATES PATENTS 2,363,695 11/1944 Ruppik164/82X 2,564,337 8/1951 Maddex... 164/283X 2,564,723 8/1951 Rossi164/283 3,338,297 8/1967 Foldessy..... 164/282X 3,416,591 12/1968 Babelet al.. 164/85 2,590,311 3/1952 I-Iarter 164/283 N v- -p 3b we 2,601,6156/1952 Jordan 164/282 3,353,584 11/1967 Atkin 164/89 FOREIGN PATENTS482,839 7/1953 Italy.

Primary Examiner-Charles W. Lanham Assistant ExaminerR. Spencer AnnearAttorney-McGlew & Toren ABSTRACT: A continuous casting operation forforming bars is comprised of pouring molten steel into the inlet end ofa mold chamber in which the bar is formed and from which the bar iswithdrawn while it still has a liquid core. Within the mold chamber themolten steel forms a liquid level, and a first section of the chamberhaving converging walls commencing at the liquid level and extendsdownwardly for applying a sym metrical cooling effect to the moltensteel and, at the same time, constraining the outer surface of the baras it contracts and develops a solidified outer layer or crust. Theconstraint afforded by the walls of the first section prevents adistortion in the shape of the bar as it begins to solidify. From thefirst section of the chamber the bar passes through a second sectionwhere the cooling operation is continued but without any constraint sothat a gap develops between the bar and the walls of the chamber. Afterpassage through the second section the bar may be withdrawn from theoutlet end of the chamber or, alternatively, it may pass through anothersection wherein increased cooling is effected and then, if necessarythrough a further section where the cooling effect is decreased as inthe second section. From the outlet the bar is guided to rollers whichwithdraw it from the mold chamber. Before reaching the rollers, coolingmeans, such as water sprays, are provided for cooling the casting. Themold chamber is provided with means for circulating a coolant fluid forremoving heat from the molten steel within the mold chamber.

METHOD OF CONTINUOUSLY CASTING BARS FOR PREVENTING DISTORTION DURINGSOLIDIFICATION OF THE BARS SUMMARY OF THE INVENTION The presentinvention is directed to a method of and apparatus for continuouslycasting a bar by pouring molten steel into a cooled open-ended mold andwithdrawing the ,bar from the mold while it still has a liquid core andaffording further cooling of the bar after its exits from the mold.

When bars of rectangular cross Section are continuously cast there is apronounced tendency, which is difficult to control, for the bars todistort angularly and to develop cracks.

In order to compensate for these defects which adversely affect both thequality of the casting as well as any further processing which may beperformed on it, it has been known to pass the cast bars between closelyset corner rollers after they have left the mold. This arrangement hasbeen successful, to some extent, in squaring off the sides of thecasting.

in continuous casting operations, it has also been proposed to usetapered molds for increasing the rate of heat removal and therebyincrease the casting rate. The tapered arrangement of the mold is formedby walls converging toward its outlet end and intended to compensate forthe shrinkage rate in the bar which is cooled as it passes through themold. However, because the shrinkage rate in themold is not a linearfunction of its length tests performed with such molds have shown thatthe casting is not positively guided until it reaches the end of themold, and as a result, the formation of peaked corners on the casting isonly partly obviated. Moreover, increasing the length of the taper sothat thecasting is guided over a greater distance within the mold failedto provide the desired results, and, as a matter of fact, merely causedthe casting to rupture because of the increased friction developed.

In any event the above described steps are attempts merely at correctingthe resulting defects in the cast bars and do not remove or correct thecauses which resulted in the defects. It has been ascertained thatpurely geometrical design measures cannot prevent the casting fromdeveloping the undesired peaked cross section-which is caused by thegeneration of asymmetrical stresses in the solidifying outer layerresulting from nonuniform cooling within the mold.

Metallographic investigations have disclosed that in continuously castingots which displayedthe peaked corner effect the finely crystallineouter layer or crust which was spontaneously solidified at the cooledwall'of the mold is of nonuniform thickness, that is, the layer may bethicker on one corner that on the oppositely disposed corner. Moreover,examination of the shells of continuous castings in which the liquidcore has run out, revealed that the nonuniformities in the thickness ofthe spontaneously formed outer layer determine the progress ofsolidification which, because of this formation, continues in the sameirregular pattern. These differences in thickness of the solidifiedcrust are the direct cause of the irregular and uncontrollablewithdrawal of the outer layer from the mold wall when the castingshrinks during further solidification. As the casting shrinks from themold wall it is usual for two opposite corners to remain in contact withthe wall for a longer period than the remaining portions of the outerlayer. Because these opposed corners remain in contact with the moldwall, solidification at these points is greatly advanced and theshrinkage below the liquid metal surface in the mold usually begins atthese corners. This corner shrinkage causes the original right angulardisposition of adjacent sides to become acute and to push the morehighly cooled corners into closer contact with the cold walls of themold. As a consequence, the other corners. which originally had shrunkfrom the mold wall, are pulled even further away. Accordingly, minorirregularities are formed in the pattern of the initial solidificationof the bar which eventually lead to angular distortion and to the develoment of fissures or cracks in its surface.

Therefore, it is a primary object of the present invention to assuresymmetrical conditions of stress during the initial stage ofsolidification of the casting as it passes through the mold for avoidingangular distortion and crack formations.

Another object of the invention is to provide a mold con struction inwhich the casting can be properly cooled and constrained during aportion of its passage through the mold.

Still another object of the invention is to provide an arrangement ofguiding means for directing the casting from the mold to the rollerswhich withdraw it from the mold.

Moreover, another object of the invention is to incorporate coolingmeans with the guiding means at the outlet end of th mold for effectingfurther cooling of the casting.

A further object of the invention is to provide a method for cooling thecasting as it passes through the mold while first providing constraintfor its exterior surface and then withdrawing the constraint. Thesesteps may be repeated where necessary for properly shaping the bar andavoiding any asymmetrical development of stresses which would tend todistort the bar.

In the method of the present invention, the molten steel is cooled andits outer layer, which forms a solidified crust, ini

tially is constrained during the passage of the casting through themold, subsequently the cooling operation is continued, however, theconstraint is removed from the casting permitting a gap to developbetween the interior surface of the mold and the exterior surface of thecasting. Where necessary the steps of cooling first with and thenwithout constraint of the exterior surface of the casting for providingan increased and then a decreased cooling effect may be repeateddepending on the various factors which affect the casting operation.

In the apparatus of the present invention vertically extending wallsform an open-ended mold chamber having an inlet opening at its upper endand an outlet opening at its lower end and arranged to have a moltensteel liquid level near its inlet opening. The first section of the moldchamber commencing at the liquid level has walls which convergedownwardly for exerting constraint on the metal as a solidified layer isformed by contact between the metal and the cooled walls of the chamber.The converging or tapered arrangement of the walls affords a smallertransverse cross section for the casting within the mold chamber thannormally would be the case where the casting is allowed to shrinkwithout constraint. Due to the constraint afforded in this first sectionmore increased heat removal operation is effected than would be possibleif the walls did not converge and the casting were allowed to cool andcontract from the walls of the mold. The second section of the moldchamber has walls of a lesser converging attitude than in the firstsection, and preferably the walls are arranged in parallel relationship.In this section, the casting continues to be cooled and to contractforming a gap between its outer surface and the juxtaposed surface ofthe mold chamber. Rollers are spaced outwardly from the mold forengaging the surface of the casting and withdrawing it from the mold.Between the rollers and the outlet a plurality of guide members areprovidedinterspersed with cooling means, such as water sprays, forfurther cooling the casting.

in an alternative arrangement, a third section and, if necessary, afourth section may be added to the mold chamber. ln the third sectionthe walls are again arranged in converging relationship so that anincreased cooling effect is provided in a manner similar to thatachieved in the first section. The fourth section may be added toprovide an effect similar to that afforded in the second section withthe walls arranged in substantially parallel relationship so that thebar continues to cool but with a reduced cooling effect.

Therefore, the method and apparatus of the present invention overcomesthe problems experienced in the prior art by effecting a symmetricalcooling effect on the casting while initially exerting sufficientconstraint to prevent the formation of a gap between the outer surfaceof the casting and the mold wall. Subsequently, the cooling of thecasting is continued but without constraint since the outer layer hasbeen adequately formed in the first section of the mold chamber.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which there are illustrated and described preferredembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a vertical sectional view of a continuous casting plantincluding apparatus embodying the present invention; and

FIG. 2 is an enlarged sectional view of another embodiment of a moldsimilar to the one disclosed in FIG. 1, which illustrates anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As illustrated in FIG.I, molten steel is poured from a tundish 1 into the open upper end 2a ofa water-cooled mold 2. The molten steel is continuously cast into a baras it passes downwardly through the mold 2 exiting from its lower end2b. During the passage of the molten steel through the mold an outerlayer 3a is solidified, however, it still retains a molten or liquidcore 3b as it issues from the outlet end 2b of the mold 2. Spaceddownwardly from the outlet end 2b of the mold are rollers 4 whichwithdraw the continuously cast bar from the mold. Guide means 5 aredisposed between the outlet end 2b and the rollers for directing the barto the rollers. Nozzles 6 are positioned between the guide means 5 forcooling the bar by spraying its outer surface with water. A jacket 7 isdisposed about the mold and forms an annular chamber for circulating acoolant fluid through the mold.

Within the upper end of the mold 2 the molten steel forms a liquid metalsurface 10. Commencing at the liquid metal surface 10 the mold isdivided into an upper first zone 11 and a lower second zone 12. Withinthe first zone 11 the opposed walls converge forming a downwardlydecreasing transverse cross section. The downward taper of the walls inthe first zone affords a cross section having a slightly smaller areathan would be developed in the casting by normal shrinkage within thissame zone. Since the cross section provided by the converging walls issmaller. it affords a constraint on the solidified outer layer or crustof the casting which would not ordinarily take place if the walls werenot properly tapered.

The walls within the first zone ll of the mold chamber hold the outerlayer of the casting tightly with a uniform pressure on all sides. Thecombination of the arrangement of the walls in the first zone and thesymmetrical cooling effect achieved by circulating a coolant fluidthrough the chamber formed between the mold 2 and the jacket 7 make itimpossible for a gap to develop between the outer surface of the castingand the juxtaposed walls of the mold chamber. However, the encirclingpressure exerted on the casting must not be so great as to developfrictional forces which cannot be overcome in withdrawing the castingfrom the mold. Excessive friction may cause the solidified outer layerto rupture and this, in turn, may result in a break through of theliquid metal core or a complete cleavage of the leading end from theremainder of the casting. It has been found that the shrinkage on eachside of the cross section in the first zone 11 of the mold is between0.4 percent and 0.8 percent. Since no gap can develop in the first zonethe entire peripheral surface of the casting is Kept in close contactwith the adjacent mold walls. This arrangement insures a high heattransfer rate between the surface of the casting and the mold wallssubmitting the solidified surface layer of the casting to asymmetrically distributed cooling effect. In other words, all fourcomers of the casting will have approximately the same temperature andthe unavoidable temperature gradient from the comers to the center ofeach adjoining side of the cross section is approximately the same.

As a result the stresses generated by these temperature gradients aresymmetrical with respect to the center of the cross section and do notgive rise to any angular distortion. Further, due to the uniform coolingeffect achieved, the thickness of the spontaneously solidified layer onthe exterior of the casting is symmetrical whereby furthersolidification within the casting occurs in a symmetrical manner.

The temperature gradient from the corners to the centers of the faces ofthe casting can be further reduced by covering the outer surface of themold wall in the region of the corners with an insulating material toreduce the transfer of heat at these locations.

The length of the first zone 11 within the mold chamber depends uponseveral casting parameters, such as the casting rate, the dimensions ofthe cross section, the quality of the cast steel and the like. Forcasting a 10 mm. square section bar at a casting rate of about 3 metersper minute, the length of the first zone 11 may be 200 mm.

If the increased cooling effect achieved in the first zone werecontinued along the entire length of the mold, the temperature gradientin the solidified layer would be excessive and would tend to cause theformation of small surface fissures. As is well known, such surfacefissures are widened by the intense cooling effect in the secondarycooling zone and may even result in the entire casting becoming uselessor being exposed to considerable scarfing losses.

In order to avoid these undesirable consequences a second zone 12affording a less intense cooling effect follows the first zone 11 in thepath of the casting through the mold chamber. The lesser cooling effectprovided in the second zone as compared to that within the first zoneoccurs because the casting is not constrained within this second zone.With the absence of any constraint, a gap 13 develops between thesurface of the casting and the adjacent mold walls. Preferably, in thesecond zone the opposed walls are arranged in parallel relationship. Ifit is desirable to obtain a larger gap 13 between the casting and theinterior surface of the mold this can be accomplished by increasing thecross section of the mold within the second zone, this can beaccomplished by arranging the walls in diverging relationship whichtends to increase the reduction in the cooling effect. Additionally, byvarying the termal conductivity of the material forming the mold, in thesecond zone 12 the cooling effect can also be reduced.

It has been known that the guide means 5 located at the outlet end ofthe mold 2 are liable to suffer distortion from radiant heat emanatingfrom the casting, and that this distortion effects the maintenance ofthe gap in the mold. In other words, the width of the gap around theperiphery of the casting and hence the cooling effect may vary. Toprevent this distortion from developing, it is preferred to providerollers 15 as the guide means for the casting at the outlet 2b from themold. Alternatively, cooling plates may be arranged in place of therollers to provide accurate alignment of the casting in relation to themold.

Alternatively, a better maintenance of the required gap between the moldand the casting in the zone 12 may be achieved by utilizing the mold 2which is illustrated in FIG. 2. The mold 2 has a first zone 11 and ashorter second zone 12' than the second zone 12 illustrated in FIG. 1.

A third zone 16 is provided at the outlet end of the second zone 12' inwhich an increased cooling effect, relative to that provided in zone12', is achieved by disposing the opposed walls in convergingrelationship. However, the reduction in cross section within zone 16 isless than that provided in zone 11 since the thickness of the surfacelayer and the mechanical strength of the casting developed in itspassage through the mold to the third zone 16 does not permit the samedegree of deformation as occurs in zone 11. Excessive deformation wouldincrease the friction with the undesirable consequences which havealready been described. Following the zone 16, another zone 17, similarto zone 12', may be provided. However, in molds of shorter length thefinal zone 17 may be omitted.

The zone 16 is afforded not only for maintenance of a uniform gap width,but also for the purpose of increasing the casting rate by the additionof a further increased cooling of the casting. Since in the second zone12' the temperature gradient in the outer layer toward thesolidification contour can flatten out because of the lesser coolingeffect provided in that zone, the increased cooling effect in zone 16can be provided without risk of producing inhomogenities in the casting.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the inventiveprinciples, it will be understood that the invention may be embodiedotherwise without departing from such principles.

We claim:

1. A method of continuously casting bars of square cross sectioncomprising the steps of continuously pouring molten steel into one endof an open-ended laterally enclosed longitudinally extending mold memberand establishing a liquid surface therein spaced from the oppositeoutlet end thereof, dividing the interior of the mold membertransversely of its longitudinal direction into a first zone havingoppositely disposed surfaces convergently tapering from the liquid sur-.face toward the outlet end and a second zone having oppositely disposedsurfaces arranged in parallel relationship extending from the downstreamend of the first zone toward the outlet end with the longitudinallyextending dimension of the first zone being such that it extends throughthe portion of the mold member where the casting normally contracts fromthe surface of the mold member, and continuously removing the cast barhaving an exterior solidified layer and a liquid core from the outletend of the mold member at such a rate that the solidified layer ismaintained in contact with the tapered mold walls in said first zone andout of contact with the mold walls in said second zone so that a uniformsymmetrical intense cooling effect takes place within the first zone anda lesser cooling effect takes place within the second zone.

2. A method as set forth in claim 1, comprising the step of guiding thebar from the outlet of the mold member.

3. A method as set forth in claim 2, comprising the step of cooling thebar as it exits from the outlet of the mold member.

4. A method as set forth in claim 3, comprising the step of engaging thesurface of the bar at a location spaced exteriorly from the outlet ofthe mold member for withdrawing the bar from the mold member.

5. A method as set forth in claim 1, comprising the step of increasingthe cooling effect within the mold member after a gap has been permittedto develop between the mold member and the bar.

6. A method as set forth in claim 5, comprising the step of decreasingthe cooling effect within the mold member after increasing the coolingeffect.

2. A method as set forth in claim 1, comprising the step of guiding thebar from the outlet of the mold member.
 3. A method as set forth inclaim 2, comprising the step of cooling the bar as it exits from theoutlet of the mold member.
 4. A method as set forth in claim 3,comprising the step of engaging the surface of the bar at a locationspaced exteriorly from the outlet of the mold member for withdrawing thebar from the mold member.
 5. A method as set forth in claim 1,comprising the step of increasing the cooling effect within the moldmember after a gap has been permitted to develop between the mold memberand the bar.
 6. A method as set forth in claim 5, comprising the step ofdecreasing the cooling effect within the mold member after increasingthe cooling effect.